Aerosol sampler



B. Y. H. LIU ETAL AEROSOL SAMPLER July 14, 1970 2 Sheets-Sheet 1 FiledMay 29, 1967 34 FIG. 1

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July 14, 1970 Filed May 29, 1967 B. Y. H. LIU ETAL 3,520,172

AEROSOL SAMPLER 2 Sheets-Sheet 2 5/ 52 Z VCORONA DCB/AJ FILaE INVENTOR.

BENJAMIN KH.L/u BY KENNETH 71 WHITE) MMMW,

Arron/vs):

United States Patent 3,520,172 AEROSOL SAMPLER Benjamin Y. H. Liu andKenneth T. Whitby, Minneapolis,

Minn., assignors to The Regents of the University of Minnesota,Minneapolis, Minn., a corporation of Minnesota Filed May 29, 1967, Ser.No. 641,896 Int. Cl. G01n 31/00 US. CI. 7328 17 Claims ABSTRACT OF THEDISCLOSURE A two stage electrostatic aerosol sampler having a chargerand a separate precipitator for collecting micron and sub-micron sizedaerosol particles on any flat collecting surface. The charger has alooped corona tungsten wire located in the aerosol passage leading tothe precipitator. A DC biased alternating voltage is applied to the wireto provide pulses of positive ions to impart an electrical charge to theaerosol particles without precipitating the aerosol particles. Theprecipitator is a non-conductive box having a removable cover forming achannel in which the charged aerosol particles flow under the influenceof a suction pump. The collecting surface, as microscope slides, arelocated on the bottom of the channel over a plate on the bottom of thecover. A pulsed voltage is applied to the plate. The voltage is of amagnitude so that the precipitating velocity of the aerosol particles isin a direction perpendicular to the direction of air flow through thechannel whereby all of the particles are collected uniformly on thecollecting surface.

BACKGROUND OF INVENTION Aerosol samplers of conventional design, as theinertia impactor, electrostatic precipitator or thermoprecipitator havesevere limitations when used to obtain samples of aerosols for sizingand counting in light or electron microscopes where quantitative data isrequired. The limitations of these samplers stems from the fact that theaerosol sample collected is usually not distributed uniformly over thecollecting surface and the presence of a considerable amount of sizeclassification of particles over the collecting surface. When sizeclassification is present, the measured size distribution of the aerosolparticles may not be representative of the aerosol particles in theiroriginal suspended state. In conventional electrostatic precipitatorswhere particles are simultaneously charged and precipitated in a coronafield, the collecting surface for particles must also conduct the largecorona current. Under these conditions a non-conductive collectingsurface would acquire sufficient electrical charge to repel the ions aswell as the charged aerosol particles. The result is that aerosolparticle precipitation becomes impossible.

SUMMARY OF INVENTION The invention relates to an apparatus and methodfor obtaining a uniform deposit of aerosol particles on any flatcollecting surface that is particularly suitable for use with electronmicroscope grids, microscope slides, cover slips, glass slides and othersurfaces with high electrical resistivity. The sampler is a two stageelectrostatic apparatus capable of sampling aerosol particles anddepositing them uniformly over a relatively large area onto any type offlat collecting surface. The deposit is quantitative so that absoluteparticle concentration of the aerosol particles can be determined. Thesampler comprises a combination charger and precipitator havingconnecting flow passages. A steady aerosol fiow is maintained throughthe passages by a suction pump attached to the discharge outlet of theprecipitator passage. The inlet end of the charging passage receives theaerosol particles for delivery to a charging region. In the chargingregion, the aerosol particles are exposed to positive ions for impartingan electrical charge on the aerosol particles. The positive ions areproduced by an ion generator located in the charging passage. The iongenerator has a corona wire to which an AC charging voltage is appliedto intermittently obtain a corona producing pulses of positive ions. Thecharged aerosol particles are distributed uniformly in the entire volumeof the precipitating region of the channel where they are subjected to apulse voltage applied for a short duration of time during which time allcharged particles are deposited uniformly over the collecting surface.During the time when the electric field is zero, another volume ofcharged aerosol particles move into the precipitating region. A secondpulse voltage deposits these particles. This cycle is repeated until thedesired number of deposits have been collected on the collectingsurfaces.

IN THE DRAWINGS FIG. 1 is a longitudinal sectional view of theprecipitator of the aerosol sampler;

FIG. 2 is a plan view taken along line 22 of FIG. 1 with the top memberremoved and the charger shown in section;

FIG. 3 is a sectional view taken along the line 33 of FIG. 1;

FIG. 4 is an enlarged sectional view of the charger shown in FIG. 2;

FIG. 5 is an end view taken along line 5-5 of FIG. 4; and

FIG. 6 is a voltage diagram of the voltage applied to the charger.

Referring to the drawings, there is shown in FIG. 2 the two stageelectrostatic aerosol sampler of this invention indicated generally at10. The sampler has a charger 11 and a precipitator 12 providingseparate charging and precipitating regions permitting the use ofcollecting surfaces with high electric resistivity, as glass microscopeslides. The aerosol indicated by arrows 13 flows into charger 11 whereit is charged with positive ions. The charger 11 is of a type capable ofcharging aerosol particles without collecting or precipitating thecharged particles. The charged aerosol particles flow into theprecipitator along with air carrying the aerosol particles. A suctionpump 14 coupled to the outlet of the precipitator draws the aerosolparticles at a steady rate through the charger and into theprecipitator.

Precipitator 12 has a box-shaped body 16 closed with a cover 17 formingan elongated flow channel 18. The volume of channel 18 is theprecipitating region of the sampler. A seal 19 around the periphery ofthe walls of the body cooperates with the cover to prevent the entranceof foreign material into channel 18. Thumb screws 34 threaded on uprightstuds hold the cover on the body. The body 16 and cover 17 may be formedof material having high electrical resistivity, as a plastic. Channel 18extends between an entrance chamber 20 and an exit chamber 21. Anupright perforated metal plate 22 separates the entrance chamber fromthe entrance of channel 18. In a similar manner, an upright metalperforated plate 23 separates the exit of channel 18 from exit chamber21. The metal plates function to maintain a uniform flow and evendistribution of the aerosol particles in channel 18. To minimize theaccumulation of charged particles in entrance chamber 20, the chamber islined with metal Walls 24 maintained at zero potential. The uprightplates 22 and 23 are also maintained at zero potential. A connector 26,as a flexible tube, couples charger 11 to body 12 so that the chargedaerosol particles are delivered to entrance chamber 20.

The bottom of channel 18 has a flat metal plate 27 J connected to aground '28. Plate 27 functions as an electrode and a support for thesampling surface 33, as electron microscope grids, microscope slides andcover slips. Located above plate 27 and secured to the bottom of cover17 is an electrically conductive plate 29 connected by a suitable lineto a power source 31 capable of generating a pulse signal 32. Plate 29covers the entire width of channel 18 and is spaced from upright plates22 and 23. The pulse signal or voltage has a square wave formillustrated at 32. As an example of the pullse signal 32, the electricfield is zero for a 3 second period during which time the entire volumeof the precipitating volume of channel 18 is filled with charged aerosolparticles. A 4200 volt pulse is then applied for 1.5 seconds duringwhich time all the charged aerosol particles are deposited uniformlyover the lower collecting surface 33. Under these conditions, theaerosol flow rate in channel 18 is sufficiently high to permit theprecipitating region to be completely filled with charged aerosolparticles during the 3 second filling period. The total aerosol sampleis dependent on the number of cycles of operation and the volume ofchamber 18 over the area of the collecting surface 33. The sample volumeis independent of the aerosol flow rate since it depends only on thearea of the collecting surface, the height of channel 18 above thecollecting surface and the number of sampling cycles.

Referring to FIGS. 4 and 5, there is shown charger 11 for charging theareosol particles without collecting or precipitating the particles.Sampler 11 has a body 36 comprised of tubular members 36A and 36Bsecured together at a right angular relationship forming an inletpassage 37 and a charging passage 38. Members 36A and 36B are one-halfinch copper fittings. One end of charging passage 38 is closed with acollar 39 slidably supporting a cylindrical support 41 for an electroderod 42. Support 41 has an outer metal tube 43 slidably supported incollar 39 and a non-conductive cylindrical member 44 within metal tube43. The electrode rod 42 extends longitudinally through member 44. Theforward end of rod 42 projects from member 44 and carries a loopedcorona wire 46. The wire 46 is a fine wire having a loop shape capableof taking large currents. Wire 46 is 0.0025 cm. diameter tungsten wire.The opposite end of rod 42 is coupled to a power source 47 generating acharging voltage 48. An example of the charging voltage is an AC voltagein the form of a 60 c.p-.s. sine wave with a peak amplitude of 800 v.FIG. 6 shows the wave form for the charging voltage 48 having a shiftedbase line. The AC voltage is subjected to a DC bias 49 sufiicient tomove the peaks 51 of the voltage wave above the voltage necessary toaccomplish corona in the area of wire 46. This causes pulsed orintermittent corona.

In use, with the charging voltage 48 applied to the charger 11, theaerosol particles 13 move into the charger through the inlet passage 37and forms a moving annular sleeve of aerosol particles about cylindricalsupport 41. As the sleeve of aerosol particles move past corona wire'46, the particles are subjected to positive ions. The result is that acharge is imposed to the aerosol particles. An AC charging voltage isused in order to charge the aerosol particles during a limited portionof the cycle with a pulse or burst of ions. This mode of chargingminimizes the loss of aerosol particles which would otherwise occur inthe charging process. The charged particles flow into the entrancechamber 20 through the perforations in upright plate 22 which uniformlydistributes the charged aerosol particles in the precipitating region ofthe sampler. With the application of the pulse voltage to plate 29, allcharged particles are deposited uniformly over the lower collectingsurface 33. The precipitating voltage is of such a magnitude that theprecipitating velocity of the aerosol particles is in a directionperpendicular to the direction of air flow through channel 18 so thatall the charged aerosol particles are collected over the bottom area ofthe channel. During the time 4 when the electric field is zero, theentire volume of channel 18 is filled with another volume of chargedaerosol particles. The next voltage pulse precipitates these chargedaerosol particles on the collecting surface 33. The total aerosolparticle sample is taken after a desired number of cycles have beencompleted.

In terms of the method of sampling aerosol particles, the inventioncomprises the steps of providing a steady flow of aerosol particles in apassage having a charging region separated from a precipitating region.During the flow of the aerosol particles they are first exposed topositive ions in a charging region. The aerosol particles are subjectedto pulses or bursts of positive ions to provide the aerosol particleswith a charge without collecting or precipitating the particles. Thecharged particles then move into a precipitating region above a particlecollecting surface 33. The charged particles are then precipitated ontothe collecting surface by subjecting the aerosol particles to a pulsedvoltage of a square wave form. This procedure is repeated until thetotal aerosol particle sample is taken.

This invention is based at least in part upon work done under a contractor grant from the United States Government.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

1. A two stage aerosol sampler comprising in combination an aerosolparticle charger having a charging passage for exposing aerosolparticles to ions to impart an electrical charge to the particles, aprecipitator coupled to the charger for receiving charged aerosolparticles, said precipitator having a channel forming a separateprecipitating chamber for charged aerosol particles and a supportadapted to hold at least one member having 2. particles collectingsurface, an inlet chamber at one end of the precipitating chamber and anoutlet at the other end of the precipitating chamber, said chargerconnected to the precipitator with the charging passage connected to theinlet chamber whereby the charging passage is separate and spaced fromthe precipitating chamber, an electrode means spaced from and locatedover said support, and means for applying a pulse precipitating voltageto the electrode means to deposit said charged aerosol particles in theprecipitating chamber on the collecting surface.

2. The sampler of claim 1 wherein said charger has a corona wire locatedin the charging passage and means for applying a voltage to said wire ofa magnitude to cause corona.

3. The sampler of claim 2 wherein said voltage is an AC voltage of amagnitude to cause intermittent corona.

4. The sampler of claim 3 wherein said voltage has an additional DCbias.

5. The sampler of claim 1 including means for uniformly distributing thecharged aerosol particles in the precipitating chamber.

6. The aerosol sampler of claim 1 wherein said electrode means forapplying the pulse precipitating voltage to the charged aerosolparticles comprises a plate along the top of the precipitating chamberof said channel and circuit means for applying the pulse precipitatingvoltage to said plate whereby the charged aerosol particles areuniformly deposited on the collecting surface.

7. The aerosol sampler of claim 6 wherein said precipitating voltage isa pulsed voltage to uniformly deposit the charged aerosol particles onthe collecting surface.

8. The aerosol sampler of claim 6 wherein said pulsed voltage has asquare wave form.

9. The aerosol sampler of claim 6 wherein said precipitatin g voltage isa DC voltage.

1.0. A method of sampling aerosol particles comprising the steps of:providing a steady flow of aerosol particles through a first passage,exposing the flowing aerosol particles to ions in the first passage toplace an electrical charge on the aerosol particles moving the chargedparticles from the first passage through an inlet chamber into aseparate precipitating chamber spaced from the first passage,maintaining the walls of the inlet chamber at zero electrical potential,precipitating the charged aerosol particles in the precipitating chamberonto a collectingsurface on one wall of the chamber by subjecting thecharged aerosol particles in the precipitating chamber of the passage toa pulse precipitating voltage.

11. The method of claim including: intermittently producing ions in thefirst passage with an AC charging voltage.

12. The method of claim 10 wherein the charged aerosol particles in theprecipitating chamber are subjected to a pulse DC precipitating voltage.

13. The method of claim 10 wherein the aerosol particles in theprecipitating chamber are subjected to a single voltage pulse touniformly deposit the charged aerosol particles on the collectingsurface.

14. An ionizer comprising a body having a passage with an inlet openingand outlet opening directing flowing'fluid, an elongated cylindricalmember having an electrical conductor located in said passage, meanslocated outside of said passage mounting said cylindrical member on saidbody along the longitudinal axis of said passage, said conductor havingcorona wire loop means at said end located entirely in said passagebetween the inlet opening and outlet opening, and circuit means forapplying an AC voltage of a magnitude to cause intermittent corona atsaid wire loop means thereby subjecting said flowing fluid to pulses ofions.

15. The ionizer of claim 14 wherein said poltage has anadditional DCbias.

16. The aerosol sampler of claim 6 including a removable cover, saidplate secured to said cover whereby the member having the particlecollecting surface can be removed from the sampler.

17. An aerosol sampler comprising in combination: an aerosol particlecharger having a body, a passage in said body, an inlet opening and anoutlet opening in communication with said passage, for directing aerosolparticles through said passage, and an elongated electrical conductorlocated in said passage between the inlet opening and the outletopening, means mounting said conductor on said body, said conductorhaving a corona wire means located in said passage between the inletopening and the outlet opening, means for applying a DC biased ACvoltage of a magnitude to cause intermittent corona to said corona wiremeans to expose the aerosol particles flowing through the passage toions to impart an electrical charge to the aerosol particles, aprecipitator comprising a body having side walls, bottom wall and coveraround a channel forming a separate precipitating region, means forremovably securing the cover to the body thereby enclosing theprecipitating region, said channel having an entrance chamber and anexit chamber on opposite ends of the precipitating region, an inletopening into the entrance chamber, an exit opening into the exitchamber, means for separating the entrance chamber and the exit chamberfrom the precipitating region for uniformly distributing the chargedaerosol particles in the precipitating region, electrically conductivewalls in said entrance chamber, said exit opening of the charger beingconnected with the inlet passage of the precipitator, means in fluidcommunication with the exit opening of the precipitator to move theaerosol particles through the charger and the precipitator, an electrodeplate means on the cover along the top of the precipitating region, andcircuit means for applying a pulsed DC square wave precipitating voltageto said plate whereby the charged aerosol particles are deposited on acollecting surface supported on the bottom wall.

References Cited UNITED STATES PATENTS 768,450 8/1904 Hardie -155 X1,358,031 11/1920 Smith 55-150 X 1,358,032 11/1920 Smith 55-1231,934,923 11/1933 Heinrich 55-139 X 2,097,233 10/1937 Meston 55-152 X2,336,625 12/1943 Milton -1 55-139 X 2,484,202 10/1949 Wintermute 73-282,857,978 10/1958 Lenger 55-151 X 2,868,318 1/1959 Perkins et al. 55-1512,949,168 8/ 1960 Bergstedt 55-152 3,027,970 4/1962 Mueller 55-129 X3,035,445 5/1962 Evans et al. 73-4215 3,157,479 11/1964 Boles 55-154 X3,181,285 5/1965 Tepolt et al. 55-138 FOREIGN PATENTS 287,648 9/ 1915Germany.

657,376 3/1938 Germany.

145,477 5 1921 Great Britain.

381,631 10/ 1932 Great Britain.

546,617 7/ 1942 Great Britain.

795,006 5/ 1958 Great Britain.

HARRY B. THORNTON, Primary Examiner D. E. TALBERT, JR., AssistantExaminer US. Cl. X.R.

532 3 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 35Z0 172 Dated July 14 1970 Inventor(s) Benjamin Y. H. Liu and Kenneth T.Whitby It is certified that error appears in the above-identified patentand that said Letters Patent are hereby corrected as shown below:

Column 3 Line 10 "pullse" should be-pulse Column 3 Line 28 "areosol"should be -aerosol Column 4 Line 36 "particles" should bepartic1e-Column 5 Line 33 "poltage should be-voltage Column 6 under ReferencesCited United States Patents "2 ,949 ,168" should be- 2 ,949,l67

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